1. Field of the Invention
The present invention relates to a method of efficiently preparing a toner for electrostatic charge image development having excellent characteristics including charging properties, preferably used in image formation in the electrophotographic method, a toner for electrostatic charge image development prepared by the method, an electrostatic charge image developer containing the toner for electrostatic charge image development, and an image forming method using the toner for electrostatic charge image development.
2. Description of the Related Art
Currently, methods for visualizing image information via an electrostatic charge image, such as the electrophotographic method are widely used in various fields. In the electrophotographic method, an electrostatic charge image is formed on a photosensitive member by a charging process, an exposing process, and the like, and is then visualized by developing the electrostatic image with a developer containing toner particles, and by a transfer process, a fixing process, and the like.
As the developer, a two-component type developer containing toner particles and carrier particles, and a one-component type developer containing magnetic toner particles or nonmagnetic toner particles, are known. The toner particles in the developer are, in general, produced by a kneading pulverizing method. The kneading pulverizing method is a method for producing desired toner particles by melting and kneading a thermoplastic resin and the like together with a pigment, a charge controlling agent, a releasing agent such as a wax and the like, then cooling the mixture, finely pulverizing the molten kneaded product, and classifying the same. To toner particles produced by the kneading pulverizing method, inorganic/organic fine particles can be further added to the surface as needed in order to improve the flowability, the cleaning properties, and the like.
In the case of toner particles produced by the kneading pulverizing method, in general, the shape thereof is amorphous, and the surface composition thereof is not homogeneous. Although the shape or the surface composition of the toner particles may change slightly according to their pulverizing properties of the material and the conditions of the pulverizing process, it is difficult to intentionally control them to a desired degree. In particular, in the case of toner particles produced by the kneading pulverizing method using a material with a high pulverizing properties, it is often the case that the particles become finer or form a different shape due to various kinds of mechanical force such as shearing force in the developing device. As a result, problems arise such as with a two-component type developer, the finer toner particles adhere to the carrier surface to accelerate charge deterioration in the developer, or with a one-component developer, the particle size distribution is enlarged so that the image quality is deteriorated by the finer toner particles being scattered, or the developing properties deteriorate due to the toner shape change.
In cases where the toner particle shape is amorphous, problems remain such as even if a flowability adjuvant is added thereto, the flowability is insufficient to prevent fine particles of the flowability adjuvant moving to concave portions of the toner particles and being buried therein by mechanical forces such as shearing force to lower the flowability, or deteriorate the developing properties, the transfer properties, the cleaning properties, and the like as time passes. Moreover, if such a toner is recovered by a cleaning treatment to be returned to the developing device for reuse, the problem of it being liable to deteriorate the image quality arises. In order to prevent these problems, it is conceivable to add more flowability adjuvant, however, problems such as the generation of black dots on the photosensitive member or particle scattering of the flowability adjuvant arise in this case.
On the other hand, in the case of a toner containing a releasing agent such as wax, the releasing agent may be exposed on the toner particle surface depending on the combination with a thermoplastic resin. In particular, in cases involving a toner comprising a combination of a resin endowed with elasticity due to a high molecular weight component making it not easily pulverized, and a brittle wax such as polyethylene, polyethylene exposure is often observed on the toner particle surface. Although such a toner is advantageous in terms of its releasing properties at the time of fixation, and cleaning of untransferred toner from the photosensitive member, problems arise such as since the polyethylene on the surface of the toner particles drops away from the toner particles due to mechanical forces such as shearing force in the developing device and easily transfers to the developing roll, the photosensitive member, the carrier, and the like, these members can be easily contaminated to ruin their reliability as developers.
Under such circumstances, recently, an emulsion polymerization aggregation method has been proposed as a means for preparing a toner with a particle shape and a surface composition intentionally controlled in Japanese Patent Application Laid Open (JP-A) Nos. 63-282752 and 6-250439. The emulsion polymerization aggregation method includes a mixing process where a coloring agent dispersion, prepared by dispersing a coloring agent in a solvent, is mixed with a resin dispersion, formed from an emulsion polymerization, an aggregating process where aggregate particles corresponding to the size of the toner particles are formed, and a fusion process where the aggregate particles are fused through being heated. According to the emulsion polymerization aggregation method, by selecting the heating temperature conditions, the toner shape can be optionally controlled from the amorphous to the spherical.
However, in the case of the emulsion polymerization aggregation method, since the degree of dispersion of the pigment in the toner particles is low with a scattered distribution, and the size of dispersed particles is large compared with the kneading pulverizing method, the colorizing ability of the toner particles obtained is low and thus problems arise in terms of color reproductivity, OHP transmission properties, and the like, particularly in case where a color toner is thus obtained.
In the conventional emulsion polymerization aggregation method, in general, toner particles are obtained by mixing a resin particle dispersion containing an ionic surfactant and a pigment dispersion containing an ionic surfactant of the opposite polarity, so as to form aggregated particles of the toner size by generating hetero aggregation, and fusing the aggregated particles by heating the same to a temperature over the glass transition point of the resin comprising the resin particles.
In this case, however, since the charge of the particles are neutralized and the hetero aggregation is generated at the time of mixing of the resin particle dispersion and the pigment dispersion, it takes time from mixing both dispersions until the liquid mixture becomes homogeneous so that pigments with a neutralized charge are aggregated at the same time, and thus the degree of dispersion of the pigment in the toner is lessened. As a result, the problem arises that the coloring properties of the obtained toner is insufficient.
On the other hand, in order to maintain and perform the toner functions stably under various kinds of mechanical stresses in the electrophotographic process, it is necessary to restrain the releasing agent exposure on the toner particle surface, to improve the surface hardness of the toner particles, and to improve the smoothness of the toner particle surface. Although the releasing agent may cause various problems when it is exposed on the toner particle surface, it is preferable that it exists in the vicinity of the toner particle surface in consideration of the toner functions at the time of fixation.
These days, a higher image quality is called for, and particularly in color image formation, there has been a marked trend to smaller sized toner particles in order to realize high resolution images. However, even if a smaller particle size is achieved, if the conventional toner particle size distribution remains as it is, problems such as the carrier or the photosensitive member becoming contaminated or the toner scattering become significant due to the existence of toner at the finer side of the particle size distribution so that it is difficult to realize a high image quality and a high reliability at the same time. In order to realize a high image quality and a high reliability at the same time, it is necessary to sharpen the particle size distribution of the toner as well as reduce the size of the particles.